The invention relates to a refrigerant dryer, in particular a compressed air refrigerant dryer for drying a gaseous fluid while cooling the gaseous fluid using a refrigerant. The invention also elates to a method for cooling a gaseous fluid in a refrigerant dryer, in particular a compressed air refrigerant dryer.
Refrigerant dryers are known per se. Reference is made to the document European patent application publication EP 1 434 023 A2 as merely one example. By the term cold drying, a method known per se is understood in general as well as according to the present invention, in which the condensable components are removed from a flow of gas by cooling the gas flow below the respective pressure dew point. The term “pressure dew point” is correspondingly understood as the temperature to which the gaseous fluid can be cooled without liquid condensing out. Cold dryers are used in particular for drying compressed air by cooling the compressed air flow charged with water vapor, partially condensing out the water vapor by the cooling process, and subsequently separating the water that has condensed out. Fields of application for “dried” compressed air are, for example, freeze prevention and corrosion protection in compressed air ducts, compressed air-driven tools and other similar applications.
It is, however, problematic to provide an energy-efficient control of the cooling capacity of a refrigerant dryer, since this control has to be effected in adaptation to variable pressure fluid volume flows, pressure fluid moistures and/or pressure fluid temperatures. Numerous methods are already known for controlling the cooling capacity of a refrigerant dryer, e.g., controls with hot gas bypass valves or controls with speed control of the refrigerant compressor. While the hot gas bypass control has a relatively poor energy efficiency, it is the limited range of control for the speed control which is problematic in most cases.
Furthermore, there are control methods which are based on switching the refrigerant compressor ON/OFF, wherein the pressure fluid temperature has to be kept as constant as possible at the condensate separator inflow irrespective of the ON/OFF switching operations of the refrigerant compressor, in order to obtain a pressure dew point which is as constant as possible.
For aiding or even enabling such an ON/OFF control of the refrigerant compressor, the provision of a cold accumulator is already known, which may be in the form of sand according to the state of the art. European Patent EP 0 405 613 B1 even proposes in this context the use of moist sand for further increasing the capacity of the cold accumulator.
Energy efficiency in the present context is to be understood as a favorable relationship between the electrical energy used and the amount of pressure fluid obtained, with an almost constant maintaining of a specified pressure dew point being assumed. An ON/OFF control in an expedient structural implementation is highly efficient but problematical with respect to maintaining a constant temperature of the pressure fluid at the condensate separator inflow.
The circuits of the refrigerant compressor are triggered in the ON/OFF control depending on the pressure fluid temperature measured at the condensate separator inflow or on other physical parameters directly or indirectly related thereto (e.g., evaporating pressure, temperature in the accumulator, temperature in the pressure fluid-refrigerant agent-heat exchanger, pressure dew point of the pressure fluid). In some cases, even several of these parameters are evaluated simultaneously and/or in a combination or weighting depending on the operating state for determining the switchpoints.
In all cases, an actually undesired hysteresis of the pressure fluid temperature has to be accepted at the condensate separator inflow, which should be minimized to a reasonable degree as far as possible, while at the same time limiting the operating cycles. This hysteresis is substantially determined by the following factors:
(1) operating cycles of the refrigerant compressor. The number of switching operations is critical to or limiting of the service life of the refrigerant compressor and increases with a decreasing hysteresis of the switchpoints;
(2) capacity of the cold accumulator;
(3) temperature gradients between the evaporating refrigerating agent, the thermal accumulator mass and the pressure fluid flow when charging or discharging the cold accumulator. These result from the heat flows to be exchanged and the construction-contingent heat transitions and heat transfers. The maximum exchanging heat flows are on an order of magnitude of the cooling capacity, with the accumulator on the one hand being forced to take up the entire cooling capacity when the refrigerant compressor is switched on under no pressure fluid flow and is forced to output it again when the refrigerant compressor is switched off under a full pressure fluid flow.
In order to keep this undesired temperature hysteresis as low as possible, despite limited refrigerant compressor operating cycles, large capacities of the cold accumulator are necessary at simultaneously good heat transfers (i.e., low temperature differences) between the evaporating refrigerant agent, the thermal accumulator mass within the cold accumulator and the pressure fluid flow, which poses structurally contradictory requirements and hence is problematic.
Since the spatial temperature gradients have an oppositely-directed progression when the cold accumulator is charged and discharged, the effects of the temperature differences will add up in the temperature hysteresis of the pressure fluid at the condensate separator inflow. In addition, large temperature differences during heat transfer are basically disadvantageous to the energy efficiency since they are associated with a high “energy production.”
To reduce the temperature differences, large and efficient heat exchanger surfaces are desirable between the evaporating refrigerant agent, the thermal accumulator mass and the flowing pressure fluid. In combination with the likewise required large capacity of the cold accumulator, this poses problems as to the structural space and arrangement of the components and heat exchanger surfaces. The problem hence includes finding a structural configuration by which large efficient heat exchanger surfaces, short heat conduction paths having a high thermal conduction coefficient, and large or voluminous cold accumulators may be obtained simultaneously with a small structural space and expenditure.
To solve this problem, the already mentioned European Patent EP 0 405 613 B1 proposes that the accumulator mass disclosed therein be embedded in the gaps of finned tube packets in the form of moist sand, the packets being interspersed by sets of tubes in which flow the pressure fluid and the evaporating refrigerant respectively. Therein, an acceptable heat transfer is achieved between the evaporating refrigerant and the pressure fluid through the metallic heat conduction paths, an acceptable size for the accumulator mass, and an embedding of the accumulator mass in direct spatial proximity and at an acceptable heat transfer both to the evaporating refrigerant agent as well as to the flowing pressure fluid. The larger these prior art cold dryers have to be dimensioned, the less favorable the cost structure becomes as a result of the necessarily large and expensive heat exchangers, which moreover require expensive raw materials such as copper and aluminum.